Donald Danforth Plant Science Center, St. Louis, Missouri
Deng, Y., Mao, G., Stutz, W., Yu, O. Generation of Composite Plants in Medicago truncatula used for Nodulation Assays. J. Vis. Exp. (49), e2633, doi:10.3791/2633 (2011).
Similar to Agrobacterium tumerfaciens, Agrobacterium rhizogenes can transfer foreign DNAs into plant cells based on the autonomous root-inducing (Ri) plasmid. A. rhizogenes can cause hairy root formation on plant tissues and form composite plants after transformation. On these composite plants, some of the regenerated roots are transgenic, carrying the wild type T-DNA and the engineered binary vector; while the shoots are still non-transgenic, serving to provide energy and growth support. These hairy root composite plants will not produce transgenic seeds, but there are a number of important features that make these composite plants very useful in plant research. First, with a broad host range,A. rhizogenes can transform many plant species, especially dicots, allowing genetic engineering in a variety of species. Second, A. rhizogenes infect tissues and explants directly; no tissue cultures prior to transformation is necessary to obtain composite plants, making them ideal for transforming recalcitrant plant species. Moreover, transgenic root tissues can be generated in a matter of weeks. For Medicago truncatula, we can obtain transgenic roots in as short as three weeks, faster than normal floral dip Arabidopsis transformation. Overall, the hairy root composite plant technology is a versatile and useful tool to study gene functions and root related-phenotypes. Here we demonstrate how hairy root composite plants can be used to study plant-rhizobium interactions and nodulation in the difficult-to-transform species M. truncatula.
The following protocol has been used to generate hairy root composite plants in a model legume species M. truncatula. Similar protocols have been adapted for at least eight plant species 1-4. We used M. truncatula hairy root composite plants to study gene functions in root and nodule development. The protocol was separated into four sections: 1) preparing plant materials; 2) generating hairy root composite plants; 3) symbiotic Rhizobia infection; and 4) transgenic root identification. We used the binary vector containing the green fluorescent protein (gfp) gene as a reporter for screening transgenic root in composite plants 3. Compared to antibiotics-based selection, GFP-based screening is fast, easy, and inexpensive. In our construct, the ER-expression-optimized gfp gene is driven by the super ubiquitin promoter, which has strong constitutive GFP signals in transgenic roots, allowing easy distinction between transgenic and non-transgenic roots.
1. Preparing Plant Materials
2. Generating Hairy Root Composite Plants
3. Symbiotic Rhizobia (Sinorhizobium Meliloti) Infection
4. Transgenic Root Identification
5. Representative Result
In our experiment, the new hair roots regenerated from the explants in 2-3 weeks after A. rhizogenes innoculation. Under the UV-microscope, transgenic roots carrying the gfp gene show strong green fluorescence (Fig 1). The amount of regenerated roots and the portion of the GFP positive roots depend on the conditions of explants and the growth environment of composite plants.On average, 25% of the roots produced were transgenic hairy roots 3.To increase the transformation efficiency, 1) the plastic cover doom, as shown on the video, is enough to maintain the humidity in the growth tray for the first few days, and plants only require little water in the first few days. Excessive watering is detrimental to hairy root formation; 2) the concentration of Agrobacterium innoculant is important for transformation. Excessive amount of cells are not helpful to hairy root formation or nodule formation. For nodule formation, the watering solution needs to be nitrogen-free, otherwise, few nodules will form.
The hairy root composite plants can be generated using cotyledons or entact seedlings as the starting material. Only minor modifications of the above protocol are ncessary to generate hairy roots from other tissues 8. Importantly, each hairy root is an independent transformation event. Therefore, the phenotypes observed in one composite plant are the sum of several transforamtion events that needed to be confirmed by repetitions in multiple composite plants
Figure 1. A. Transgenic roots can be sorted out from hairy roots. B. Nodules were formed in the hairy root composite plants. We placed 4-week-old M. truncatula hairy root plants under the UV-microscope and the GFP positive roots can be easily identified. (Red arrow: GFP root; Yellow arrow: non-GFP root)
|Stock||g/200ml||ml stock/ liter|
|Micronutrients||g per liter|
Table 1. Nitogen-free nutrition solution
Table 2. yeast extract mannitol medium (per liter)
Generating hairy root composite plant is a quick and easy method to obtain large quantities of transgenic material for many dicot species. Although this method cannot produce transgenic seeds, it can produce transgenic materials in a few weeks. The method is especially suitable for plants that have difficulty establishing tissue cultures or generating stable transformants. Over the years, we have used this technology to study gene functions, promoter functions, microRNAs, root and lateral root development, defense and abiotic stress responses, nodulation and other symbiotic processes, hormone responses, metabolic profiling, gene profiling, proteomic analysis, and other biological processes. The protocol is robust and replicable.
No conflicts of interest declared.
The authors would like to thank Dr. Chris Taylor (Ohio State University) for introducing hairy root technology to our lab and Drs. Senthil Subramanian (South Dakota State University), Juan Zhang (Ludong University, Shandong China) for improving the protocol. We also thank Dr. Hao Cheng (Nanjing Agriculture University) for helps in making this video. This work is supported in part by grants from DOE (DE-SC0001295), NSF (MCB-0923779) and USDA (2010-65116-20514) to O.Y